Benzyl metal compounds are important synthetic intermediates for construction of a carbon skeleton. Hitherto, there are many known synthesis methods of such compounds, as exemplified below. (1) A method in which a benzyl ether is reacted with lithium, to give benzyl lithium (J. Org. Chem., 26, 3723 (1961)); (2) A method in which benzyl halides are reacted with metal zinc or cadmium, to give benzyl zinc halides or benzyl cadmium halides (Compt. Rend., 245, 2054 (1957)); (3) A method in which, first, a Grignard reagent prepared from benzyl halides and metal magnesium is converted to a benzyl mercury compound or a benzyl tin compound, and then the resultant compound is treated with an alkyl lithium, to give benzyl lithium (J. Organomet. Chem., 2, 431 (1964)); (4) A method in which toluene is reacted with an alkyl lithium, in the presence of a specific ligand (Organometallics, 4, 2117 (1985)); (5) A method in which an alkyl lithium compound is reacted with another metal alkoxide, and then the resultant alkyl metal compound is reacted with toluene (Angew. Chem. Int. Ed. Engl., 32, 1501 (1993)); (6) A method in which phenyl sodium is reacted with toluene, under heat reflux (J. Am. Chem. Soc., 62, 1514 (1940)); (7) A method in which phenyl sodium is reacted with toluene, in the presence of tetrahydrofuran, at room temperature (JP-A-48-75551 ("JP-A" means unexamined published Japanese patent application)).
However, all of these methods have some respective problems. That is, in the method (1), only 1 equivalent of benzyl lithium can be generated from 2 equivalents of expensive lithium. In the method (2), the yield is low. The route of the method (3), via a benzyl mercury compound or a benzyl tin compound, is to remove dibenzyl (Wurtz coupling product), which is a by-product at the Grignard reaction, but the method (3) cannot be used in industrial production because harmful heavy metal wastes are discharged. The methods (4) and (5) are not economically profitable, because an expensive alkyl lithium compound is used, and moreover it is necessary to add a ligand that is used to accelerate the reaction, in an amount of not less than 1 equivalent. In the method (6), the reaction of phenyl sodium with toluene is so slow at room temperature that the operation of heat reflux is necessary, as well as that the heat reflux also causes the problem that tar is generated due to thermal decomposition of phenyl sodium. The method (7) was developed to attempt to overcome the drawbacks of the method (6). Consequently the method (7) has an advantage that the reaction can be carried out at room temperature, but actually this method is not suitable for industrial production, from a viewpoint of separation and recovery of a solvent, because it is necessary to add water-soluble tetrahydrofuran to phenyl sodium, in an amount of 20 to 300 wt % based on phenyl sodium.
On the other hand, examples of a method of preparing 4-phenyl-1-butenes include (a) a method in which benzyl magnesium halides are reacted with allyl halides (e.g. J. Am. Chem. Soc., 55, 699 (1933)), (b) a method in which allyl magnesium halides are reacted with benzyl halides (e.g. Bull. Soc. Chim. Fr., 43, 1326 (1928)), (c) a method in which phosphorusylides are reacted with carbonyl compounds according to the Wittig reaction (e.g. JP-A-3-294271), (d) a method in which phenethyl magnesium halides are reacted with vinyl halides, in the presence of a nickel catalyst (e.g. U.S. Pat. No. 5,030,784), and (e) a method in which allyl halides are reacted with benzyl sodium that was obtained by reacting phenyl sodium with toluene under heat reflux (e.g. J. Chem. Soc., 1975 (1950)).
However, in the methods (a) and (b), synthesis of the Grignard reagent itself is difficult, due to competition with the Wurtz coupling. In the methods (c) and (d), expensive catalysts are used, and a great deal of effort is required to separate and purify the products. Consequently, it is difficult to say that these methods are not satisfactory for industrial practice. The method (e) necessitates the operation of heat reflux in order to give benzyl sodium, and at this time thermal decomposition of phenyl sodium occurs, so that reduction of the yield is unavoidable. For example, the yield of 4-phenyl-1-butene that is given by reacting with allyl chloride is only about 51 to 72%.
The present invention was made in order to solve the above-described problems, and an object of the present invention is to provide a novel method of preparing benzyl metal compounds.
Further, another object of the present invention is to provide a method that can prepare 4-phenyl-1-butenes from benzyl metal compounds, in which the benzyl metal compounds are prepared, at a high purity and a high yield, by the use of relatively easily available raw materials, catalysts, and the like, without heat reflux and the like.
Other and further objects, features, and advantages of the invention will appear more fully from the following description.